In the past decade considerable effort has gone into the development and characterization of perfluorosulfonic acid polymer electrolytes such as Nafion. These efforts have shown that polymer electrolyte membranes (PEM) offer a number of advantages over conventional electrolytes when used in electrochemical devices such as fuel cells and water electrolyzers.
U.S. Pat. No. 5,525,436, entitled "Proton Conducting Polymers", the disclosure of which is incorporated herein by reference, discloses the use of polymer electrolyte membranes, for example polybenzimidazole (PBI) doped with phosphoric acid, which are capable of conducting protons at temperatures of up to at least 200.degree. C. These membranes, therefor, avoid prior art problems related to dehydration of the membrane. Further, disadvantages due to poisoning of the electrode catalysts and fuel crossover are overcome by the novel polymer electrolyte membranes disclosed in the patent. The preparation of the membranes of the patent involves first casting the membrane film from an appropriate solution, such as dimethyl acetamide (DMAc), and then doping the film with the desired acid constituent. Conductivity in the range of from 0.01 to 0.04 S/cm for temperatures from 130.degree. C.-190.degree. C. and water vapor partial pressures up to 1 atmosphere were recorded for H.sub.3 PO.sub.4 doped PBI films.
It has now been discovered that the conductivity of polymer electrolyte membranes of the type discussed above may be significantly and unexpectedly enhanced by preparation of the membrane from a solution of the doped polymer in an acid. For example, a PBI film doped with H.sub.3 PO.sub.4 and prepared from trifluoroacetic acid (TFA) solution exhibits conductivity measured at 0.04-0.08 S/cm, as compared to the lower conductivity measured for PBI membranes cast from DMAc and subsequently doped.
It has further been discovered that the economics of membrane production can be reduced by casting the PBI membranes directly from a casting solution containing H.sub.3 PO.sub.4 and including trifluoroacetic acid (TFA) as a solvent.
It is, therefore, an object of the subject invention to provide a method for casting a solid polymer electrolyte membrane which does not suffer from known problems associated with catalyst stability and activity, and which demonstrates enhanced conductivity.
It is another object of the invention to provide a casting solution from which a solid polymer electrolyte membrane which is suitable for use in direct methanol fuel cells without exhibiting high methanol permeability resulting in loss in efficiency due to methanol crossover can be produced by direct casting methods.